1979 and the destruction of relations between Iran and the United States

The United States was active in helping Shah Mohammad Reza Pahlavi maintain the rule he established in 1941 via underhanded activity in the country (the CIA overthrowing Prime Minister Mohammed Moassadeq in 1953, for instance) and outright financial support. His pro-Western positions were seen as a good thing for the US and its economy and people so the government chose to throw their full support behind him, a decision that would eventually come back to haunt them.
The Islamic Revolution happened in 1979 and it took on a deeply anti-American sentiment in large part because the United States was seen as having propped up the man the country overthrew. Ever since that time, the US has been a convenient boogeyman for the leaders of Iran to point to when they need a global villain to rally their people to a cause that offers little actual benefit to them.